crypto/rand: skip TestAllocations on plan9

crypto/rand: skip TestAllocations on plan9

The function crypto/rand.read allocates on Plan 9, so this test
would always fail.

Fixes #69873

Change-Id: I8d661cb868c32f6985d0926f9ae7b30204cafbf4
Reviewed-on: https://go-review.googlesource.com/c/go/+/624015
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>